Aluminum-zinc-magnesium-copper alloy wrought product

ABSTRACT

An aluminum alloy flat rolled product having improved strength and fracture toughness, the aluminum base alloy comprised of 1.95 to 2.5 wt. % Cu, 1.95 to 2.5 wt. % Mg, 8.2 to 10 wt. % Zn, 0.05 to 0.25 wt. % Zr, max. 0.15 wt. % Si, max. 0.15 wt. % Fe, max. 0.1 wt. % Mn, the remainder aluminum and incidental elements and impurities.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. Ser. No. 11/083,517,filed Mar. 18, 2005, which is a continuation-in-part of U.S. Ser. No.10/662,835, filed Sep. 15, 2003, which claims the benefit of U.S.Provisional Application No. 60/412,200, filed Sep. 21, 2002,incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates to Al—Zn—Mg—Cu alloys and more particularly itrelates to Al—Zn—Mg—Cu extrusions and flat rolled products and themethod of making the same for use in aircraft applications. Further, theinvention relates to Al—Zn—Mg—Cu alloy products having improved strengthand fracture toughness.

Existing Al—Zn—Mg—Cu alloys can have relatively high strengths atmoderate corrosion resistance and moderate damage tolerance or fracturetoughness. Such alloys and methods of obtaining properties are set forthin the patents. For example, U.S. Pat. No. 4,863,528 discloses a methodfor producing an aluminum alloy product and the resulting product havingimproved combinations of strength and corrosion resistance. The methodincludes providing an alloy consisting essentially of about 6-16% zinc,about 1.5-4.5% magnesium, about 1-3% copper, one or more elementsselected from zirconium, chromium, manganese, titanium, vanadium andhafnium, the total of said elements not exceeding about 1%, the balancealuminum and incidental impurities. The alloy is then solution heattreated; precipitation hardened to increase its strength to a levelexceeding the as-solution heat treated strength level by at least about30% of the difference between as-solution heat treated strength and peakstrength; subjected to treatment at a sufficient temperature ortemperatures for improving its corrosion resistance properties; andagain precipitation hardened to raise its yield strength and produce ahigh strength, highly corrosion resistant alloy product.

U.S. Pat. No. 5,221,377 discloses an alloy product having improvedcombinations of strength, density, toughness and corrosion resistance,said alloy product consisting essentially of about 7.6 to 8.4% zinc,about 1.8 to 2.2% magnesium, about 2 to 2.6% copper and at least oneelement selected from zirconium, vanadium and hafnium present in a totalamount not exceeding about 0.5%, preferably about 0.05 to 0.25%zirconium, the balance aluminum and incidental elements and impurities.The alloy product, suitable for aerospace applications, exhibits highyield strength, at least about 10% greater yield strength than its7X50-T6 counterpart, with good toughness and corrosion resistanceproperties typically comparable to or better than those of its 7X50-T76counterpart. Upper wing members made from this alloy typically have ayield strength over 84 ksi, good fracture toughness and an EXCOexfoliation resistance level of “EC” or better, typically “EB”.

U.S. Pat. No. 4,477,292 discloses a three-step thermal aging method forimproving the strength and corrosion resistance of an article comprisinga solution heat treated aluminum alloy containing zinc, magnesium,copper and at least one element selected from the group consisting ofchromium, manganese and zirconium. The article is precipitation hardenedat about 175° to 325° F., heat treated for from several minutes to a fewhours at a temperature of about 360° to 390° F. and again precipitationhardened at about 175° to 325° F. In a preferred embodiment the articletreated comprises aluminum alloy 7075 in the T6 condition. The method ofthe invention is easier to control and is suitable for treating articlesof greater thickness than other comparable methods.

U.S. Pat. No. 5,108,520 discloses an aging process forsolution-heat-treated, precipitation hardening metal alloy whichincludes first underaging the alloy, such that a yield strength belowpeak yield strength is obtained, followed by higher aging for improvingthe corrosion resistance of the alloy, followed by lower temperatureaging to strength increased over that achieved initially.

U.S. Pat. No. 5,560,789 discloses AA 7000 series alloys having highmechanical strength and a process for obtaining them. The alloyscontain, by weight, 7 to 13.5% Zn, 1 to 3.8% Mg, 0.6 to 2.7% Cu, 0 to0.5% Mn, 0 to 0.4% Cr, 0 to 0.2% Zr, others up to 0.05% each and 0.15%total, and remainder Al. Either wrought or cast alloys can be obtained,and the specific energy associated with the DEA melting signal of theproduct is lower than 3 J/g.

U.S. Pat. No. 5,312,498 discloses a method of producing analuminum-based alloy product having improved exfoliation resistance andfracture toughness which comprises providing an aluminum-based alloycomposition consisting essentially of about 5.5-10.0% by weight of zinc,about 1.75-2.6% by weight of magnesium, about 1.8-2.75% by weight ofcopper with the balance aluminum and other elements. The aluminum-basedalloy is worked, heat treated, quenched and aged to produce a producthaving improved corrosion resistance and mechanical properties. Theamounts of zinc, magnesium and copper are stoichiometrically balancedsuch that after precipitation is essentially complete as a result of theaging process, no excess elements are present. The method of producingthe aluminum-based alloy product utilizes either a one- or two-stepaging process in conjunction with the stoichiometrically balancing ofcopper, magnesium and zinc.

U.S. Pat. No. 4,711,762 discloses an improved aluminum base alloyproduct comprising 0 to 3.0 wt. % Cu, 0 to 1.5 wt. % Mn, 0.1 to 4.0 wt.% Mg, 0.8 to 8.5 wt. % Zn, at least 0.005 wt. % Sr, max. 1.0 wt. % Si,max. 0.8 wt. % Fe and max. 0.45 wt. % Cr, 0 to 0.2 wt. % Zr, theremainder aluminum and incidental elements and impurities.

U.S. Pat. No. 1,418,303 discloses an improved aluminum alloy consistingof copper about 0.1% to any amount below 3%, titanium about 0.1% toabout 2%, zinc about 6% to about 16%, iron (present as an impurity ofcommercial aluminum) preferably not exceeding 0.6%, silicon (present asan impurity of commercial aluminum) preferably not exceeding 0.4%, otherelements (impurities) preferably not exceeding 0.4%, remainder aluminum.

U.S. Pat. No. 2,290,020 discloses an improved aluminum alloy having theternary compound of aluminum, zinc and magnesium present in an amountranging from about 2% to 20%, the preferred range being between about 3%and 15%. At room temperature the ternary compound goes into solidsolution in aluminum alloys in an amount of about 2%. The percentage insolid solution increases at high temperatures and decreases uponcooling, the excess precipitating out.

U.S. Pat. No. 3,637,441 discloses an aluminum base powder metallurgyalloy article having an improved combination of high-transverse yieldstrength and high-stress corrosion cracking resistance. The alloycontains the basic precipitation hardening elements zinc, magnesium andcopper plus dispersion strengthening elements iron and nickel. It mayadditionally contain chromium and/or manganese. The alloy is prepared byatomization of a melt of the elements, hot-working, solution heattreating, quenching and artificial aging. Components of the alloy inpercent by weight are, in addition to the aluminum, from at least 6.5 to13 zinc, 1.75 to 6 magnesium, 0.25 to 2.5 copper, 0.75 to 4.25 iron and0.75 to 6 nickel, up to 3 manganese and up to 0.75 chromium. The iron tonickel ratio is from 0.2:1 to 2.0:1.

U.S. Pat. No. 5,028,393 discloses an Al-based alloy for use as slidingmaterial, superior in fatigue resistance and anti-seizure propertyconsisting, by weight, of 1-10% Zn, 1-15% Si, 0.1-5% Cu, 0.1-5% Pb,0.005-0.5% Sr, and the balance Al and incidental impurities.

U.S. Pat. No. 6,315,842 discloses a mold for plastics made of a rolled,extruded or forged AlZnMgCu aluminum alloy product >60 mm thick, andhaving a composition including, in weight %: 5.7<Zn<8.7, 1.7<Mg<2.5,1.2<Cu<2.2, Fe<0.14, Si<0.11, 0.05<Zr<0.15, Mn<0.02, Cr<0.02, withCu+Mg<4.1 and Mg>Cu, other elements <0.05 each and <0.10 in total, theproduct being treated by solution heat treating, quenching and aging toa T6 temper.

In spite of these discloses, there is still a great need for an improvedalloy and extrusions and flat rolled products fabricated therefrom foraerospace applications having high levels of strength, corrosionresistance, fracture toughness and good resistance to fatigue crackgrowth. The subject invention provides such products.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved Al—Zn—Mg—Cualloy for extrusions, sheet or plate for use in aircrafts.

It is another object of the invention to provide an Al—Zn—Mg—Cu alloyextrusions, sheet or plate having improved fracture toughness as well ashaving high strength levels.

It is yet another object of the invention to provide a method forproducing an Al—Zn—Mg—Cu alloy extrusions, sheet or plate havingimproved strength properties, fracture toughness and resistance tofatigue crack growth.

It is still another object of the invention to provide a method forproducing an Al—Zn—Mg—Cu alloy product having improved strengthproperties, fracture toughness, good levels of corrosion resistance.

It is another object of this invention to provide aerospace structuralmembers such as extrusions from the alloy of the invention.

In accordance with these objects, there is provided a method ofproducing an aluminum alloy extrusions, sheet or plate products havingimproved strength and fracture toughness, the method comprising thesteps of providing a molten body of an aluminum base alloy comprised of1.95 to 2.5 wt. % Cu, 1.95 to 2.5 wt. % Mg, 8.2 to 10 wt. % Zn, 0.05 to0.25 wt. % Zr, max. 0.15 wt. % Si, max. 0.15 wt. % Fe, max. 0.1 wt. %Mn, the remainder aluminum and incidental elements and impurities; andcasting the molten body of the aluminum base alloy to provide asolidified body, the molten aluminum base alloy being solidified at arate between liquidus and solidus temperatures in the range of 600° to800° K. per second to provide a solidified body or ingot having a grainsize in the range of 25 to 75 μm. Thereafter, the body or ingot ishomogenized by heating in a first temperature range of 840° to 860° F.followed by heating in a second temperature range of 680° to 880° F.wherein the second temperature is higher than the first to provide ahomogenized body having a uniform distribution of MgZn₂ or ηprecipitate. The homogenized body is then extruded or rolled to providean extrusion or flat rolled product. For the extrusion, the extruding iscarried out in a temperature range of 600° to 850° F. and at a ratesufficient to maintain at least 80% of said extrusion in anon-recrystallized condition. The extrusion is solution heat treated andartificial aged to improve strength properties and to provide anextrusion product having improved fracture toughness.

The improved aluminum base alloy extrusion product can have a yieldstrength to fracture toughness ratio of 8% or greater than a similarlysized 7xxx product.

The invention also includes an improved aluminum base alloy wroughtproduct such as an extrusion, sheet or plate product consistingessentially of 1.95 to 2.5 wt. % Cu, 1.95 to 2.5 wt. % Mg, 8.2 to 10 wt.% Zn, 0.05 to 0.25 wt. % Zr, 0.05 to 0.2 wt. % Sc, max. 0.15 wt. % Si,max. 0.15 wt. % Fe, max. 0.1 wt. % Mn, the remainder aluminum andincidental elements and impurities.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart showing steps of the invention.

FIG. 2 illustrates the results of the damage tolerance (normalizeddenting speed) of the invention alloy (M703) compared to a high strength7xxx alloys (SSLLC).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, there is shown a flow chart of steps in theinvention. Generally, in the steps a molten body of Al—Zn—Mg—Cu alloy iscast at a controlled solidification rate to obtain a specific grain sizerange in the cast body. Thereafter, the cast body is homogenized undercontrolled conditions to obtain a uniform distribution of MgZn₂ or ηprecipitate. The body may be rolled to sheet or plate product or it maybe extruded in a specific rate range and temperature to obtain anextrusion product having a large portion thereof, e.g., at least 80%, ina non-recrystallized condition. The product is then solution heattreated and aged to very high levels of strength, fracture toughness andcorrosion resistance.

The alloy of the invention contains about 8.2 to 10 wt. % Zn, 1.95 to2.5 wt. % Mg, 1.95 to 2.5 wt. % Cu, 0.05 to 0.25 wt. % Zr, max. 0.15 wt.% Si, max. 0.15 wt. % Fe, max. 0.1 wt. % Mn, the remainder aluminum,incidental elements and impurities.

Preferably, the alloy contains 1.95 to 2.3 wt. % Cu, 1.95 to 2.3 wt. %Mg, 8.45 to 9.4 wt. % Zn, 0.05 to 0.2 wt. % Cr and 0.05 to 0.15 wt. %Zr. Cr can range from 0.05 to 0.08 wt. %. For purposes of retardingrecrystallization, the alloys can contain 0.01 to 0.2 wt. % Sc,preferably 0.01 to 0.1 wt. %. Such alloys when processed in accordancewith the invention possess marked improvements in yield strength andfracture toughness at acceptable or even high levels of corrosionresistance compared to conventional 7xxx alloys such as AA7075-T6, forexample. The composition of the AA 7xxx alloys are set forth in TheAluminum Association publication entitled “Registration Record ofAluminum Association Designations and Chemical Composition Limits forWrought Aluminum and Wrought Aluminum Alloys”, dated December 1993. Theterm “7xxx” means aluminum alloys containing zinc as a main alloyingingredient. AA 7075-T6 refers to AA compositional limits as registeredwith The Aluminum Association. A typical T6 aging practice for 7075 isheating at about 250° F. for 24 hours and a typical temperature range isabout 175° to 330° F. for 3 to 30 hours.

For purposes of the present invention, a molten aluminum alloy of theinvention is cast into a solidified body at a rate which provides acontrolled microstructure or grain size. Such molten aluminum alloytypically is cast in the form of billet when it is desired to produceextrusion products. Further, typically such solidified body is cast at arate of about 1 to 6 inches per minute, preferably 2 to 4 inches perminute, and typically the billet has a diameter in the range of about 1to 7 inches. For purposes of the invention, it is preferred that thesolidified body has an average grain size in the range of 25 to 100 μm,preferably 35 to 75 μm. If the alloy of the invention is cast atcontrolled rates and thermally mechanically processed in accordance withthe invention, very high tensile and compressive strengths, fracturetoughness and corrosion resistance can be obtained. That is, forpurposes of obtaining the desired microstructure for thermal mechanicalprocessing in accordance with the invention, the molten aluminum is castat a controlled solidification rate. It has been discovered thatcontrolled solidification rate of the disclosed aluminum alloy incombination with subsequent controlled thermal mechanical processingresults in extruded products having superior properties, i.e., very hightensile strength, good corrosion and dent resistance.

It should be noted that the strength of the subject aluminum alloys canbe improved by dispersion hardening or by strain hardening. Strainhardening is the result of plastic deformation and is dependent on thedegree of deformation. Dispersion hardening is produced throughformation of clusters of atoms (referred to as Guiner-Preston or GPzones). In addition, precipitation hardening can result from theformation of new phases or precipitates in the alloy which form barriersagainst dislocation movement. This can significantly increase thestrength of the alloy. In the Al—Zn—Mg—Cu alloys, new strengtheningphases include MgZn₂, also known as M or η-phase; Mg₃Zn₃Al₂ also as theT-phase; CuMgAl₂ also known as the S-phase. Strengthening resulting fromprecipitation of new phases is more effective than strengthening byformation of GP zones. However, strengthening by precipitation of newphases can have an adverse effect on damage tolerance or fracturetoughness. Usually, the greater the volume fraction in the precipitationphases, the lower is the damage tolerance. By comparison, strengtheningresulting from GP zone formation does not take place at the expense ofdamage tolerance. Thus, to provide for improved strength and damagetolerance, the present invention balances the volume fraction ofprecipitates and the volume fraction of GP zones or zinc-rich clustersin the final product while maintaining excess zinc in solution. For thepurpose of the invention the GP zones size should be in the range of 2to 35 nm and the GP zones density should be in the range of 4×10¹⁸ to5×10¹⁸ zones per cm³.

For purposes of producing billet in accordance with the invention,casting may be accomplished using a mold cooled by an air and liquidcoolant to solidify billet at a controlled rate which provides thedesired grain size or structure. The grain can have a size in the rangeof 35 to 75 μm. The air and coolant mixture used with the molds areparticularly suited for extracting heat from the body of molten aluminumalloy to obtain a solidification rate of 5° to 50° C. per second forbillet having a diameter of 1 to 6 inches. Molds using the air andcoolant mixture which are suitable for controlling the cooling rate forcasting molten aluminum alloy of the invention are described in U.S.Pat. No. 4,598,763. The coolant for use with these molds for theinvention is comprised of a gas and a liquid where gas is infused intothe liquid as tiny, discrete undissolved bubbles and the combination isdirected on the surface of the emerging ingot. The bubble-entrainedcoolant operates to cool the metal at an increased rate of heatextraction; and if desired, the increased rate of extraction, togetherwith the discharge rate of the coolant, can be used to control the rateof cooling at any stage in the casting operation, including during thesteady state casting stage.

For casting metal, e.g., aluminum alloy to provide a microstructuresuitable for purposes of the present invention, molten metal isintroduced to the cavity of an annular mold, through one end openingthereof, and while the metal undergoes partial solidification in themold to form a body of the same on a support adjacent the other endopening of the cavity, the mold and support are reciprocated in relationto one another endwise of the cavity to elongate the body of metalthrough the latter opening of the cavity. Liquid coolant is introducedto an annular flow passage which is circumposed about the cavity in thebody of the mold and opens into the ambient atmosphere of the moldadjacent the aforesaid opposite end opening thereof to discharge thecoolant as a curtain of the same that impinges on the emerging body ofmetal for direct cooling. Meanwhile, a gas which is substantiallyinsoluble in the coolant liquid is charged under pressure into anannular distribution chamber which is disposed about the passage in thebody of the mold and opens into the passage through an annular slotdisposed upstream from the discharge opening of the passage at theperiphery of the coolant flow therein. The body of gas in the chamber isreleased into the passage through the slot and is subdivided into amultiplicity of gas jets as the gas discharges through the slot. Thejets are released into the coolant flow at a temperature and pressure atwhich the gas is entrained in the flow as a mass of bubbles that tend toremain discrete and undissolved in the coolant as the curtain of thesame discharges through the opening of the passage and impinges on theemerging body of metal. With the mass of bubbles entrained therein, thecurtain has an increased velocity, and this increase can be used toregulate the cooling rate of the coolant liquid, since it more thanoffsets any reduction in the thermal conductivity of the coolant. Infact, the high velocity bubble-entrained curtain of coolant appears tohave a scrubbing effect on the metal, which breaks up any film andreduces the tendency for film boiling to occur at the surface of themetal, thus allowing the process to operate at the more desirable levelof nucleate boiling, if desired. The addition of the bubbles alsoproduces more coolant vapor in the curtain of coolant, and the addedvapor tends to rise up into the gap normally formed between the body ofmetal and the wall of the mold immediately above the curtain to cool themetal at that level. As a result, the metal tends to solidify further upthe wall than otherwise expected, not only as a result of the highercooling rate achieved in the manner described above, but also as aresult of the build-up of coolant vapor in the gap. The higher levelassures that the metal will solidify on the wall of the mold at a levelwhere lubricating oil is present; and together, all of these effectsproduce a superior, more satin-like, drag-free surface on the body ofthe metal over the entire length of the ingot and is particularly suitedto thermal transformation.

When the coolant is employed in conjunction with the apparatus andtechnique described in U.S. Pat. No. 4,598,763, this casting method hasthe further advantage that any gas and/or vapor released into the gapfrom the curtain intermixes with the annulus of fluid discharged fromthe cavity of the mold and produces a more steady flow of the latterdischarge, rather than the discharge occurring as intermittent pulses offluid.

As indicated, the gas should have a low solubility in the liquid; andwhere the liquid is water, the gas may be air for cheapness and readyavailability.

During the casting operation, the body of gas in the distributionchamber may be released into the coolant flow passage through the slotduring both the butt forming stage and the steady state casting stage.Or, the body of gas may be released into the passage through the slotonly during the steady state casting stage. For example, during thebutt-forming stage, the coolant discharge rate may be adjusted toundercool the ingot by generating a film boiling effect; and the body ofgas may be released into the passage through the slot when thetemperature of the metal reaches a level at which the cooling raterequires increasing to maintain a desired surface temperature on themetal. Then, when the surface temperature falls below the foregoinglevel, the body of gas may no longer be released through the slot intothe passage, so as to undercool the metal once again. Ultimately, whensteady state casting is begun, the body of gas may be released into thepassage once again, through the slot and on an indefinite basis untilthe casting operation is completed. In the alternative, the coolantdischarge rate may be adjusted during the butt-forming stage to maintainthe temperature of the metal within a prescribed range, and the body ofgas may not be released into the passage through the slot until thecoolant discharge rate is increased and the steady state casting stageis begun.

The coolant, molds and casting method are further set forth in U.S. Pat.Nos. 4,598,763 and 4,693,298, incorporated herein by reference.

While the casting procedure for the present invention has been describedin detail for producing billet having the necessary structure forthermal transformation in accordance with the present invention, itshould be understood that the other casting methods can be used toprovide the solidification rates that result in the grain structurenecessary to the invention. As noted earlier, such solidification can beobtained by belt, block or roll casting, DC and electromagnetic casting.

A seven inch billet of an alloy containing 8.9 wt. % Zn, 2.1 wt. % Mg,2.1 wt. % Cu, 0.11 wt. % Zr, the remainder comprising aluminum, castemploying a mold using air and water coolant, at a cooling rate of 35°to 50° F. per second provides a satisfactory grain structure forextruding and thermally mechanically processing in accordance with theinvention.

While casting has been described with respect to billet, it will beappreciated that the principles described herein may be applied to ingotor electromagnetic casting of the aluminum alloys.

Thus, after the ingot or billet is cast, it is subjected to ahomogenization treatment. Preferably, the ingot or billet is subjectedto two homogenization treatments. In the first homogenization treatment,the ingot or billet preferably is treated in a temperature range of 840°to 860° F. for a period of 6 to 18 hours. Thereafter, the ingot orbillet is then preferably subjected to a temperature range of 860° to880° F. for a period of 4 to 36 hours. In the second step, thetemperature is higher than the temperature used in the first step.Subjecting the ingot or billet to a double homogenization treatment asdescribed provides a more uniform distribution of MgZn₂ precipitate or Mor η-phase as well as zinc and chromium containing dispersoids.

After homogenization, the billet is extruded to provide an extrusionmember. For purposes of extruding, the billet is heated to a temperaturerange of 600° to 850° F. and maintained in this temperature range duringextruding. Preferably, the billet is extruded at a rate in the range of0.8 to 8 ft/min and preferably at an extrusion ratio in the range of 10to 60. These conditions are important to obtain an extrusion wherein atleast 80% and preferably 90% of the extrusion is maintained in theunrecrystallized condition. The extrusion can have an aspect ratiobetween the thinnest and thickest section of 1:4 to 1:18.

For purposes of producing flat rolled products, the ingot is hot rolledto a thickness, e.g., a thickness in the range of 2 to 12 inches. Hotrolling is usually performed in a temperature range of 750° to 810° F.for the entrance temperature and an exit temperature in the range of770° to 840° F.

After extruding or hot rolling, the product is solution heat treated byheating in a temperature range of about 870° F. to about 890° F., with apreferred temperature range being 875° to 885° F. Typical times at thesetemperatures can range from 5 to 120 minutes. The solution heat treatingshould be carried out for a time sufficient to dissolve a substantialportion of the alloying elements. That is, substantially all of thezinc, magnesium and copper is dissolved to provide a solid solution.

After solution heat treating, the product is rapidly cooled or quenchedby immersion or spraying with cold water, for example. After quenching,the extrusion, for example, may be straightened and/or stretched. Thatis, the product is straightened or leveled prior to aging to improvestrength properties.

After solution heat treating, the extrusion or flat rolled product istreated to improve properties such as strength, corrosion and fracturetoughness.

Thus, the extrusion or flat rolled product may be subject to differentthermal treatments depending on the properties desired. For example, theextrusion or plate may be subject to a single step thermal treatment toachieve high or peak strength, referred to as T6 type tempers. However,such tempers can be susceptible to stress corrosion cracking. T6 tempersare obtained by aging at a temperature range of 175° to 325° F. for 3 to30 hours. A two step aging process may be employed wherein a first agingstep is carried out at 175° to 300° F. for a period of time of 3 to 30hours, followed by a second aging step carried out at 300° to 360° F.for a period of time of 3 to 24 hours. This aging process produces anoveraged temper referred as T7x temper. This condition improves stresscorrosion cracking but can decrease strength.

To improve strength and corrosion resistance, the extrusion or flatrolled product may be subject to a three-step aging process. The agingsteps or phases include a low-high-low aging sequence. In the first orlow aging step, the product is subject to a temperature for a period oftime which precipitation hardens the extrusion to a point at or nearpeak strength. This can be effected by subjecting the product toprecipitation hardening in a temperature range of about 150° to 325° F.typically for a time between 2 to 30 hours. Then, the extrusion or plateis subject to a second treatment to improve corrosion resistance. Thesecond treatment includes subjecting the extrusion or plate to atemperature range of 300° to 500° F. for 5 minutes to about 3 hours, forexample. In the third step, the product is subject to anotherstrengthening step. The third thermal treatment includes subjecting theproduct to a temperature of 175° to 325° F. for about 2 to 30 hours.

Exfoliation corrosion (EXCO) behavior of the inventive alloy wascompared to 7075 T6511 and 7075 T76511 alloys. The American Society forTesting and Materials developed a method (ASTM G34-99) that provides anaccelerated exfoliation corrosion test for 2xxx and 7xxx series aluminumalloys. The susceptibility to exfoliation is determined by visualexamination, with performance ratings established by reference tostandard photographs. When tested in accordance with this test methodthe alloy of the invention exhibits a typical EA exfoliation corrosionrating when aged to a T76 temper. When aged to a T77 temper theinvention alloy exhibits a typical EB exfoliation corrosion rating.

While alloy has been described with respect to extrusion and plateproducts, it can find use as sheet and forged product and such iscontemplated herein.

While the invention can provide extrusion with high strength andfracture toughness, it can also be used to produce sheet and plate andeven forgings having similar properties. Plate has relatively thickgauges which can range from 2 to 12 inches, typically 3 to 8, and partsmade or machined from such plate exhibit the high levels of strength andfracture toughness. Machine parts from plate find use in the aircraft oraerospace industry and include spar, rib and web members which are usedin wing boxes of the aircraft. As sheet, the invention finds use assheet on aircraft where high damage tolerance is required, particularlyin lower wing applications. It will be understood that the high strengthand fracture toughness of the alloy parts permit thinner or less thickand therefore lighter parts, thereby saving weight on the aircraft.Further, aircraft parts machined from plate in accordance with theinvention can have properties 5 to 10% higher over alloys such asAA7050, 7010, for example, of similar parts and still maintain goodresistance to stress corrosion cracking.

Aluminum alloy 7050-T74 is often used for plate which can be machined toproduce wing spars, wing ribs, etc., for large aircraft. Aerospace andMaterials Specification AMS 4050F lists a minimum yield strength of 60KSI in the longitudinal direction for AA7050-T7451 six inch plate. Italso specifies a fracture toughness of 24 K_(IC).

Ingot for producing plate may be DC cast or electromagnetic cast. The DCcast ingots are typically scalped to remove a thin surface layer whichcan contain unwanted inclusions. Thereafter, the ingot is subject to ahomogenization treatment, as noted.

All ranges set forth herein include all the numbers within the range asif specifically set forth.

The products or members described herein in accordance with theinvention are particularly suitable for aerospace applications and findsmany uses in large aircrafts such as commercial and military aircrafts.As noted, the products can be used in wing components, tail assemblies,fuselage sections or in subassemblies or other components comprising theaircraft. That is, the aircraft assemblies can comprise a wing assemblyor wing subassembly, a center wing box assembly or subassembly, floorassembly or subassembly including seat tracks, floor beams, stanchions,cargo deck assemblies and subassemblies, floor panels, cargo floorpanels, fuselage assemblies or subassemblies, fuselage frames, wingribs, fuselage stringers and the like. Further, the products may beproduced as seamless or non-seamless tubes and used in sporting goodssuch as baseball bats. TABLE Typical mechanical properties of theinventive alloy (M703) in comparison to 7075 T6511 and 7150 T77511 forextrusions 0.249 inch thick Alloy Temper UTS, ksi YS, ksi e, % K_(q)7075 T6511 88 82 10 28 M703 T76511 97 93 10 33 M703 T77511 102 100 932.5 7150 T77511 93 89 9 27

The table illustrates the mechanical properties of the inventive alloywhen aged to a T76 and a T77 tempers.

The following examples are still further illustrative of the invention.

EXAMPLE 1

A billet of an alloy containing 8.9 wt. % Zn, 2.1 wt. % Mg, 2.1 wt. %Cu, 0.11 wt. % Zr, incidental elements and impurities, the balancealuminum, was cast into a seven inch diameter billet. The billet wascast using casting molds utilizing air and liquid coolant (availablefrom Wagstaff Engineering, Inc., Spokane, Wash.). The air/water coolantwas adjusted in order that the body of molten aluminum alloy was cast ata rate of 4 inches per minute. The as-cast structure had an averagegrain size of 35 μm. The billet was homogenized for 8 hours at 870° F.and then for 24 hours at 885° F. Thereafter, the billet was brought to atemperature of 725° F. and extruded into a hollow tube with an outsidediameter of 2.65 inch and a wall thickness of 0.080 inch.

The extrusion had a non-recrystallized grain structure. The extrusionwas solution heat treated for 25 minutes at 880° F. and quenched in awater-15% glycol solution. Thereafter, the quenched extrusion wasprecipitation hardened for 24 hours at 250° F. and then subjected to atemperature of 315° F. for 6 hours to improve corrosion resistance andyield strength properties. The extrusion was then tested for tensilestrength and yield strength and compared to AA 7075 T6. The results arereproduced in Table 1.

The extrusion was then tested for dent resistance or damage tolerance.The dent resistance test was performed by pitching balls of constantsize and weight at the extruded tube. The number of pitches to the firstdent on the extrude tube represents the dent resistance. The extrusionwas compared to a AA 7055 alloy treated in a similar fashion. The alloyof the invention is referred to as M703 and 7055 as SSLLC (see FIG. 2).Both alloys were aged identically. It will be seen from FIG. 2 that M703had superior dent resistance.

EXAMPLE 2

A billet of an alloy containing 8.9 wt. % Zn, 2.1 wt. % Mg, 2.1 wt. %Cu, 0.11 wt. % Zr, incidental elements and impurities, the balancealuminum, was cast into a seven inch diameter billet. The billet wascast using casting molds utilizing air and liquid coolant (availablefrom Wagstaff Engineering, Inc., Spokane, Wash.). The air/water coolantwas adjusted in order that the body of molten aluminum alloy was cast ata rate of 4 inches per minute. The as-cast structure had an averagegrain size of 35 μm. The billet was homogenized for 8 hours at 870° F.and then for 24 hours at 885° F. Thereafter, the billet was brought to atemperature of 725° F. and extruded into an aircraft stringer having a“T” shaped cross section and a wall thickness of 0.245 inches.

The extrusion had a non-recrystallized grain structure. The extrusionwas solution heat treated for 35 minutes at 880° F. and quenched in awater-15% glycol solution. Thereafter, the quenched extrusion wasprecipitation hardened for 24 hours at 250° F. followed by 25 to 35minutes at 380° F., then subjected to a temperature of 250° F. for 24hours. The extrusion was then tested for tensile strength and yieldstrength and fracture toughness, fatigue crack growth and compared to AA7075 T6511 and AA 7150 T77511. The results are reproduced in Table 1. Itwill be seen that the inventive alloy has superior strength and fracturetoughness when compared to AA 7075 T6511 and AA 7150 T77511. Also, theextrusion has a unique combination of tensile strength, corrosionresistance, and damage tolerance (i.e., fracture toughness and fatiguecrack growth).

Having described the presently preferred embodiments, it is to beunderstood that the invention may be otherwise embodied within the scopeof the appended claims.

1. A method of producing an aluminum alloy flat rolled product having:improved strength and fracture toughness, the method comprising thesteps of: (a) providing a molten body of an aluminum base alloycomprised of 1.95 to 2.5 wt. % Cu, 1.95 to 2.5 wt. % Mg, 8.2 to 10 wt. %Zn, 0.05 to 0.25 wt. % Zr, max. 0.15 wt. % Si, max. 0.15 wt. % Fe, max.0.1 wt. % Mn, the remainder aluminum and incidental elements andimpurities; (b) casting said molten body of said aluminum base alloy toprovide an ingot, said molten aluminum base alloy being cast at a ratein the range of 1 to 6 inches per minute; (c) homogenizing said ingot byheating in a first temperature range of 840 to 860° F. followed byheating in a second temperature range of 860° to 880° F., the secondhomogenization temperature being greater than the first temperature toprovide a homogenized ingot having a uniform distribution of ηprecipitate and zirconium containing dispersoids; (d) rolling saidhomogenized ingot to provide a flat rolled product, said rolling beingcarried out in a temperature range of 600° to 850° F.; (e) solution heattreating said flat rolled product; and (f) artificial aging said flatrolled product to provide a product having improved strength andfracture toughness.
 2. The method in accordance with claim 1 wherein thealloy contains 1.95 to 2.3 wt. % Cu.
 3. The method in accordance withclaim 1 wherein the alloy contains 1.95 to 2.3 wt. % Mg.
 4. The methodin accordance with claim 1 wherein the alloy contains 0.05 to 0.2 wt. %Cr.
 5. The method in accordance with claim 1 wherein the alloy contains8.45 to 9.4 wt. % Zn.
 6. The method in accordance with claim 1 whereinthe alloy contains 0.01 to 0.1 wt. % Sc.
 7. The method in accordancewith claim 1 wherein the alloy contains 0.01 to 0.2 wt. % Ti.
 8. Themethod in accordance with claim 1 including heating in said firsttemperature range for 6 to 18 hours.
 9. The method in accordance withclaim 1 including heating in said second temperature range for 4 to 36hours.
 10. The method in accordance with claim 1 including rapidlyquenching said flat rolled product.
 11. The method in accordance withclaim 1 wherein said solution heat treating is carried out in atemperature range of 875° to 885° F. for 5 to 120 minutes.
 12. Themethod in accordance with claim 1 wherein said artificial aging iscarried out by aging in a temperature range of 175° to 300° F. for 3 to30 hours followed by aging at 280° to 360° F. for 3 to 24 hours.
 13. Themethod in accordance with claim 1 wherein said artificial aging iscarried out by aging in a temperature range of 210° to 280° F. for 4 to24 hours followed by aging at 320° to 400° F. for 30 minutes to 14hours.
 14. The method in accordance with claim 1 wherein said artificialaging is carried out by aging in a temperature range of 150° to 325° F.for 2 to 30 hours followed by aging at 300° to 500° F. for 5 minutes to3 hours followed by aging at 175° to 325° F. for 2 to 30 hours.
 15. Themethod in accordance with claim 1 wherein said artificial aging is athree-step process wherein said first and third steps improve strengthand a second step improves corrosion resistance.
 16. The method inaccordance with claim 1 wherein said artificial aging includes aging:(i) at a low temperature above room temperature to precipitation hardensaid extrusion; (ii) at temperatures to improve corrosion resistanceproperties of said extrusion; and (iii) at lower temperatures above roomtemperature to precipitation harden said extrusion.
 17. The method inaccordance with claim 1 wherein the flat rolled product has a fracturetoughness at least 5% greater than a similar product fabricated from7075 alloy.
 18. The method in accordance with claim 1 wherein the flatrolled product has a fracture toughness at least 8% greater than asimilar product fabricated from 7075 alloy.
 19. The method in accordancewith claim 1 wherein the flat rolled product is a plate product havingthickness in the range of 2 to 12 inches.
 20. A method of producing analuminum alloy flat rolled product having improved strength and fracturetoughness, the method comprising the steps of: (a) providing a moltenbody of an aluminum base alloy comprised of 1.95 to 2.3 wt. % Cu, 1.95to 2.3 wt. % Mg, 8.2 to 9.4 wt. % Zn, 0.05 to 0.2 wt. % Cr, 0.05 to 0.15wt. % Zr, max. 0.15 wt. % Si, max. 0.15 wt. % Fe, max. 0.1 wt. % Mn, theremainder aluminum and incidental elements and impurities; (b) castingsaid molten body of said aluminum base alloy to provide an ingot, saidmolten aluminum base alloy being cast at a rate in the range of 1 to 6inches per minute; (c) homogenizing said ingot by heating in a firsttemperature range of 840° to 860° F. for 6 to 24 hours followed byheating in a second temperature range of 860° to 880° F. for 4 to 36hours wherein the second homogenization temperature is greater than thefirst temperature to provide a homogenized body having a uniformdistribution of η precipitate and zirconium and chromium containingdispersoids; (d) rolling said homogenized ingot to provide a plateproduct, said rolling being carried out in a temperature range of 600°to 850° F.; (e) rapidly quenching said plate product; (f) solution heattreating said plate product; and (g) artificial aging said product toprovide a product having improved strength and fracture toughness. 21.The method in accordance with claim 20 wherein the alloy contains 0.01to 0.1 wt. % Sc.
 22. The method in accordance with claim 20 wherein thealloy contains 0.01 to 0.2 wt. % Ti.
 23. The method in accordance withclaim 20 wherein said solution heat treating is carried out in atemperature range of 875° to 885° F. for 5 to 120 minutes.
 24. Themethod in accordance with claim 20 wherein said artificial aging iscarried out by aging in a temperature range of 175° to 300° F. for 3 to30 hours followed by aging at 280° to 360° F. for 3 to 24 hours.
 25. Themethod in accordance with claim 20 wherein said artificial aging iscarried out by aging in a temperature range of 245° to 255° F. for 6 to24 hours followed by aging at 360° to 390° F. for 5 to 120 minutes. 26.The method in accordance with claim 20 wherein said artificial aging isa three-step process wherein said first and third steps improve strengthand a second step improves corrosion resistance.
 27. The method inaccordance with claim 20 wherein said artificial aging includes aging:(i) at a low temperature above room temperature to precipitation hardensaid extrusion; (ii) at temperatures to improve corrosion resistanceproperties of said extrusion; and (iii) at lower temperatures above roomtemperature to precipitation harden said extrusion.
 28. The method inaccordance with claim 20 wherein the plate product has a fracturetoughness at least 5% greater than a similar plate product fabricatedfrom 7075 alloy.
 29. The method in accordance with claim 20 wherein saidartificial aging is carried out by aging in a temperature range of 150°to 325° F. for 2 to 30 hours followed by aging at 300° to 500° F. for 5minutes to 3 hours followed by aging at 175° to 325° F. for 2 to 30hours.
 30. A method of producing an aluminum alloy plate product havingimproved strength and fracture toughness, the method comprising thesteps of: (a) providing a molten body of an aluminum base alloycomprised of 1.95 to 2.5 wt. % Cu, 1.95 to 2.5 wt. % Mg, 8.2 to 10 wt. %Zn, 0.05 to 0.25 wt. % Zr, max. 0.15 wt. % Si, max. 0.15 wt. % Fe, max.0.1 wt. % Mn, the remainder aluminum and incidental elements andimpurities; (b) casting said molten body of said aluminum base alloy toprovide an ingot, said molten aluminum base alloy being cast at a ratein the range of 1 to 4 inches per minute; (c) homogenizing said ingot byheating in a first temperature range of 840 to 860° F. followed byheating in a second temperature range of 860° to 880° F., the secondhomogenization temperature being greater than the first temperature toprovide a homogenized ingot having a uniform distribution of ηprecipitate; (d) rolling said homogenized ingot to provide a plateproduct, said rolling being carried out in a temperature range of 600°to 850° F. to provide said plate product in a substantiallynon-recrystallized condition; (e) rapidly quenching said plate product;(f) solution heat treating said plate product; and (g) artificial agingsaid product to provide a product having improved strength and fracturetoughness.
 31. The method in accordance with claim 30 wherein the alloycontains 0.05 to 0.2 wt. % Cr.
 32. The method in accordance with claim30 wherein the alloy contains 0.01 to 0.2 wt. % Ti.
 33. The method inaccordance with claim 30 wherein the alloy contains 0.01 to 0.2 wt. %Sc.
 34. The method in accordance with claim 30 wherein said solutionheat treating is carried out in a temperature range of 875° to 885° F.for 5 to 120 minutes.
 35. The method in accordance with claim 30 whereinsaid artificial aging is carried out by aging in a temperature range of175° to 300° F. for 3 to 30 hours followed by aging at 280° to 360° F.for 3 to 24 hours.
 36. The method in accordance with claim 30 whereinsaid artificial aging is carried out by aging in a temperature range of210° to 280° F. for 4 to 24 hours followed by aging at 300° to 400° F.for 1 to 14 hours.
 37. The method in accordance with claim 30 whereinsaid artificial aging includes aging: (i) at a low temperature aboveroom temperature to precipitation harden said extrusion; (ii) attemperatures to improve corrosion resistance properties of saidextrusion; and (iii) at lower temperatures above room temperature toprecipitation harden said extrusion.
 38. The method in accordance withclaim 30 wherein said artificial aging is carried out by aging in atemperature range of 150° to 325° F. for 2 to 30 hours followed by agingat 300° to 500° F. for 5 minutes to 3 hours followed by aging at 175° to325° F. for 2 to 30 hours.
 39. An improved aluminum base alloy plateproduct consisting essentially of 1.95 to 2.5 wt. % Cu, 1.95 to 2.5 wt.% Mg, 8.2 to 10 wt. % Zn, 0.05 to 0.25 wt. % Zr, max. 0.15 wt. % Si,max. 0.15 wt. % Fe, max. 0.1 wt. % Mn, the remainder aluminum andincidental elements and impurities, said plate product having a fracturetoughness of 5% or greater and a yield strength of 8% or greater than asimilarly sized 7075 product.
 40. The alloy product in accordance withclaim 39 wherein the alloy contains 1.95 to 2.3 wt. % Cu.
 41. The alloyproduct in accordance with claim 39 wherein the alloy contains 1.9 to2.3 wt. % Mg.
 42. The alloy product in accordance with claim 39 whereinthe alloy contains 0.05 to 0.2 wt. % Cr.
 43. The alloy product inaccordance with claim 39 wherein the alloy contains 8.45 to 9.4 wt. %Zn.
 44. The alloy product in accordance with claim 39 wherein the alloycontains 0.01 to 0.2 wt. % Sc.
 45. The alloy product in accordance withclaim 39 wherein the alloy contains 0.01 to 0.2 wt. % Ti.
 46. The alloyproduct in accordance with claim 39 wherein said product is a wing spar.47. The alloy product in accordance with claim 39 wherein sand productis a wing rib.
 48. The alloy product in accordance with claim 39 whereinsaid product is an aircraft stringer.
 49. The alloy product inaccordance with claim 39 wherein said product is an aircraft floor beam.50. The alloy product in accordance with claim 39 wherein said productis an aircraft fuselage beam.